The example I used was hypothetical. The numbers I used were clearly made up, and it was used to explain that niches in small margins are not harmful to the game. Just look at racial bonuses as another example.
It is not fair to call a difference that results in a large margin a niche. At this point, its not a niche anymore, but a specializiation. Specialized tanks means that one tank can be useless in content, niched tanks means that all tanks are useful, but certain ones will perform a task that is unique to its job.
Having bad DPS isn't a niche, thats just bad tuning when it comes to DPS. Bard's niche isn't that it has lower DPS then other DPS classes, but that it brings some unique support that other classes don't bring. The DPS loss is there to balance the class in respect to other classes.
No one is asking for overpowered specialized tanks, that would obviously break the game. Racial bonuses do not break the game. When niches happen that do not overwhelmingly effect the meta such like the example I used, then the effect the niche has, (making the job unique), is healthy for the game.
Again I reiterate the point that niches should be marginally small, and not the extremely large ones you mentioned that make tanks useless in a fight. Classes clearing content 4 weeks later then others isn't because its a problem with niches, but a problem with tuning. The point in niches isn't to reinforce the idea that you need specialized tanks, but to make the jobs unique so they aren't homogenized and boring.
Again, having high or low DPS doesn't count as a niche, but having the highest fire type damage could be considered a niche. If a boss was weak to fire damage, would you bother changing your raid composition to bring fire? Not unless the difference was so large that the content is unclearable without it. (This is not what I'm arguing for.)